Hi,
mIRC simply relays WM_MOUSEMOVE messages from Windows. Windows is telling mIRC that the mouse moved, and mIRC is relaying this message. It's not a bug. There's no guarantee that a "mouse" event (in mIRC) means the mouse's coordinates changed-- it simply means that Windows told mIRC the mouse moved. It could very well have moved from coordinate X,Y to the same X,Y.